Terms used

A Test Case contains the actual testing code. It utilizes assertions etc. to report success or failure of certain operations. A test case is implemented as a single routine.

This understanding corresponds to test methods (i.e. methods decorated with [Test]) in NUnit.

A Test Fixture is a collection of test cases, accompanied by a setup and teardown routine.

This understanding corresponds to test classes (i.e. classes decorated with [TestFixture]) in NUnit.

All fixtures are compiled and linked into a single binary, which is a DLL file. This DLL file is referred to as Test Module.

A collection of one or more test modules is referred to as test suite.